This item by J.P. Green was published on January 20, 2009.
In her Wall St. Journal article, “Union Households Gave Boost to GOP’s Brown,” Melanie Trottman reports on a new Hart Research Poll:

A poll conducted on behalf of the AFL-CIO found that 49% of Massachusetts union households supported Mr. Brown in Tuesday’s voting, while 46% supported Democrat Martha Coakley…The poll showed Ms. Coakley drew more support among voters with a college education, by a five-point margin, while she lost by a 20-point margin among voters without a college degree.

Tula Connell puts it this way in her FiredogLake post, “The Working Class Has Spoken. Will Democrats Listen?” at the AFL-CIO Now Blog:

The poll, conducted by Hart Research Associates among 810 voters for the AFL-CIO on the night of the election, also found that although voters without a college degree favored Barack Obama by 21 percentage points in the 2008 election, Democratic candidate Martha Coakley lost that same group by a 20-point margin.

Connell cites another poll, noted in John Judis’s post in The New Republic:

…The Suffolk University poll in Massachusetts…singled out two white working-class towns, Gardner and Fitchburg, as bellwethers. Obama won Gardner, where Democrats hold a 3-1 registrations edge, by 59 percent to 31 percent in 2008. Brown won it by 56 percent to 42 percent. Obama won Fitchburg, with a similar Democratic edge, by 60 percent to 38 percent in 2008. Brown won it by 59 percent to 40 percent. That suggests a fairly dramatic shift among white working-class voters.

Trotman quotes Karen Ackerman, the AFL-CIO’s political action director, describing the MA senate vote as “a working-class revolt” with too many voters feeling that no one was addressing their interests. Ackerman adds:

Union voters are like any other voters, and they respond to the environment around them…What happened in Massachusetts is that working families did not see the Democratic candidate as being on their side.

The poll also provides “pretty strong evidence: that union workers may have been concerned about the ‘cadillac benefits’ tax, according to Hart Research pollster Guy Molyneux, despite President Obama’s assurance that workers in collective bargaining agreements until 2018 would be exempt. “Unions stepped up their campaign efforts for Ms. Coakley after that, but it wasn’t enough to turn the tide,” reports Trottman.
It’s important to remember that Brown’s 3 percent edge with unions in the poll is within the margin of error. In addition, 61% of the voters polled said they were picking the best candidate for Massachusetts rather than sending a message to Washington. with 33% saying the opposite. Two out of three voters who elected Mr. Brown said they wanted him to work with Democrats in Washington.
Connell argues that the Hart poll suggests ‘liberal overreaching” is not the problem:

In fact, voters were not worried about Democratic “overreach”—47 percent said their bigger concern about Democrats is that they haven’t succeeded in making needed change rather than tried to make too many changes too quickly (32 percent). Even voters for Scott Brown were more concerned about a lack of change (50 percent) than about trying to make too many changes too quickly (43 percent)…These results puts a lie to the corporate media spin that Democrats have gone “too far” in pushing a reform agenda.

But whether Brown won a small majority of union voters or not is less important than the fact that he got even close, as an outspoken opponent of health care reform endorsed by Senator Kennedy and EFCA and as an advocate of a range of anti-worker policies. Coakley’s policies were far more supportive of worker concerns across the board than were Brown’s. But, if union members in the most liberal state have not been well-informed about the progressive policies at issue, how bad was the situation for unorganized workers?
Some have speculated that Coakley may have added to the anti-worker meme by dissing the idea that she campaign at Fenway Park and taking a week-long vacation in December, at a time when many blue collar workers in the state were unemployed and struggling to pay the mortgage/rent and feed their families. Certainly such blunders don’t help, and Brown’s supporters were more than eager to publicize them as the outrage du jour.
It’s hard to avoid the conclusion that Coakley’s campaign failed to connect adequately with perhaps the largest of swing constituencies, the MA working class. The candidate and her campaign, the national and local Democratic Parties and the unions were all caught off guard by Brown’s late surge. On paper, Coakley was a good candidate, by all accounts an excellent AG, bright and progressive. But her messaging to MA workers was weak. while Brown’s was aggressively presented and resonated in the vacuum.